The invention disclosed and claimed herein pertains generally to the field of equalizers or equalization apparatuses located at the receiving end of a data transmission channel which provides an inverse model thereof. More particularly, the invention pertains to equalizer apparatuses having a very rapid convergence time, i.e., which is capable of being set up as an inverse model of a data transmission channel in a very short time period. Even more particularly, the invention pertains to equalizer apparatuses for employment with data transmission channels which are subject to fading and related frequent or continual changes in channel characteristics.
As is well known in the art, an equalizer may be considered as a type of filter capable of providing a desired output in response to a specified input. The input to an equalizer usually is data which has been distorted by its passage through a data transmission channel and the desired equalizer output is the data in its original, undistorted form. Equalizer devices, which evolved in the mid-1960's, provided inverse models of data transmission channels and were usually located at the receiving ends. When channel distorted data was coupled into an equalizer, it was inversely distorted so that the equalizer output comprises the data in undistorted condition.
In order to set up, or "train," an equalizer as an inverse transmission channel model, a sequence of prespecified training signals were coupled directly to the equalizer, so that they were received without distortion. By determining the error between the undistorted and channel distorted (i.e. transmitted) forms of successive training signals and by adjusting the equalizer in relation to such errors, the equalizer was adapted to become an inverse channel model by the time the training signal sequence concludes.
Later equalizer designs generally are assemblies of adjustable electrical components coupled to appropriate means for varying respective parameters of the components in accordance with successive training signal errors. Such means include calculation or like devices for performing a series of mathematical computations in response to each training signal error. From the equalizers discussed, it will be readily apparent that their efficiency is closely related to the length of the training signal sequence which is required to set the equalizer up, and also to the number and complexity of the mathematical computations which are required per training signal error.
Cost and complexity of fabrication may make one design more attractive than another; however, a far more important consideration is operational efficiency. Efficiency of operation is directly related to an equalizer's convergence time, (the time required to adapt an equalizer to inversely model the characteristics of a particular data transmission channel). Convergence time is especially important when the data transmission channel is subjected to continual internal and external variations in channel characteristics, and when data is to be transmitted at very high rates of speed. If an equalizer is incapable of rapidly adapting or converging to the new channel characteristics, significant amounts of transmitted data may be lost.
The invention cross-referenced above provides a significant advance in the state-of-the-art in terms of both the convergence time and an equalizer's operational efficiency. The referenced apparatus has particular utility in the operation of a communication system wherein data is transmitted at a rapid rate through a channel which is subject to frequent or continual fading. Unfortunately, the equalizer is designed as an all zero equalizer and does not have a pole-zero capability.